April 23, 1998

A weekly feature provided by scientists at the Hawaiian Volcano
Observatory.

Shhh! Don't tell there is an eruption - Eruption of 1942

The eruption of 1942 was noteworthy for several reasons. (1) It
was declared a secret so the press was not allowed to publicize the event.
(2) This was the second time lava diversion was tried on an eruption of
Mauna Loa. (3) The volcanologists were able to predict the timing and
place of the eruption.

World War II was four months old when the eruption began. Hawai`i
was observing war-time restrictions that imposed a night-time blackout.
Pele was having none of this, and the eruptions began on April 26, 1942, at
5:05 p.m. Government officials felt that advertising the eruption would
permit the Japanese military to use the bright glow as a means to guide
their warplanes under cover of darkness to wreak havoc upon Hawai`i.

The eruption, heralded by three months of elevated seismic
activity, started when fissures opened along the western rim of
Moku`aweoweo, and lava cascaded into the caldera. By early morning April
27, the summit activity began to wane. Eventually, seismic activity
migrated toward the northeast rift zone. New fissures then opened at the
junction between the summit caldera and northeast rift on April 27. Here
the dike stopped advancing at the 3,658-m (12,000 ft) level. A line of
spatter ramparts issued an `a`a flow that traveled in a north-northeasterly
direction 7 km (4 mi) away from the rift. all this activity had ended by
the early morning of April 28.

However On April 28, activity jumped far down the northeast rift
zone. This new vent system opened at 4:40 a.m., creating a 1-km-long (0.6
mi) fissure at an elevation of 2,850 m (9,350 ft), 14.5 km (9 mi) from
Mauna Loa's summit. Lava fountains propelled 70 to 90 meters (230 to 295
ft) high, producing small flows. Meanwhile the summit activity continued
to fade as the flank activity expanded. Eventually, the summit activity
ceased altogether.

By 3 p.m., the 1-km-long (0.6 mi) fissure lengthened an additional
0.5 km (0.3 mi). Fountains then were visible only from the distal ends of
the fissure system; the central vents were drowned by lava moving in from
the margins.

At 8 p.m., the eruption appeared to have begun in earnest as lava
fountains reached heights of 150 m (500 ft). The fountains fed a rapidly
moving `a`a lava flow headed in the general direction of Waiakea Uka.
Gradually, eruptive activity condensed to a few spatter ramparts at the
center of this fissure system. These vents were responsible for the bulk
of the lava production.

A few hours later, the dike propagated another 4.6 km (2.9 mi)
northeast. Here the dike changed orientation from northeast to north.
Lava issuing from these lower elevation vents appeared to be gas-poor and
more dense in character. The character of the lava caused scientists to
speculate that these "new vents" were really sites where upslope lava
entered an old tube or fracture system, only to reemerge at this lower
elevation.

On May 1, the flow front was moving at 100 to 150 m (328 to 492 ft)
per hour in the direction of Waiakea Uka. The advancing flow threatened
the `Ola`a flume, Mountain View's water source, and would ultimately
endanger the circum-island road. It was at this time that the decision was
made to utilize aerial bombing to influence the course of the lava flows.

Aerial bombing was selected as the most efficient means of
emplacing explosives at the optimal sites. The points of contact were lava
tubes and channels that fed the flow front. The Army Air Force conducted
the bombing missions and selected the sites. In all, 16 bombs weighing
between 300 to 600 pounds each were dropped. Though most appeared to hit
their mark, the bombs had little or no impact on the eruption or on the
flow's direction.

On May 4, mauka activity became more restricted to a single, large
cone edifice with vents producing lava at its center. The cone grew to a
height of 30 meters (100 ft) and lava fountains stretched to heights of 200
m (650 ft). Later in the day, lava flows accomplished what bombs could
not. These lava flows caused the central cone to collapse, resulting in
the creation of secondary flows. These secondary flows diverted the lava
from the flow front, slowing its advance, and resulted in the stagnation of
the main flow on May 7. When the eruption ended on May 9, the lava flow
extended 26 km from the main vents at 2,850 m (9,350 ft) and came within 11
km (6.8 mi) of Upper Waiakea Uka.

Eruption and Earthquake Update

Activity within Pu`u `O`o increased during the past week, and lava
covered the floor of the crater. The light from the lava cast a bright glow
onto the fume clouds. Lava continued to flow through a network of tubes
down to the seacoast, where it entered the ocean at two locations -
Waha`ula and Kamokuna. The public is reminded that the ocean entry areas
are extremely hazardous, with explosions accompanying frequent collapses of
the lava delta. The steam cloud is highly acidic and laced with glass
particles.

There were no earthquakes reported felt during the past week.

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hvowebmaster@usgs.govUpdated:
27 April 1998